I believe the proper WINTER tire size for RS6 is 225/45/18 (would be interesting to drive 255 in 4 inches of snow) - this is from our dealer here and he offered that as an option (second set of wheels).

it is the same outside diameter of tire as 255/40/18. I am sure that you will find this size in US, at least H rated. Tirerack or the others should have it. It was about $220/tire when we looked for it couple months ago.

gg